1. Field of the Invention
The present invention relates to a thermal printer head having an array of heating elements wherein the current supply to the heating elements is controlled according to a parameter representative of the temperature of the heating elements.
2. Description of the Related Art
With current thermal printer heads, the printed image density is initially low during a period immediately following the start of a print and becomes high with time as the printing process continues. This is due to the absorption of thermal energy partly by the printer head itself and partly by materials surrounding the head. Since this initial image density variation seriously degrades the quality of a printed material, proposals have been made to overcome this heat absorption problem.
One prior art proposal maintains a record of energy that has been supplied to the printer head for a period of time and uses this record as correction data to control the amount of energy subsequently supplied to the head. While this method proved successful for applications where the print ratio and its variation are both small, such as in the case of character printing, it requires a large amount of past records for printing a graphic image since its print ratio varies significantly in the directions of coordinates. Large scale integration technology is therefore necessary for implementation.
Another prior art method involves the use of a thermistor temperature sensor for producing a signal representative of the temperature of the heating elements and controlling the amount of energy supplied to them according to the detected temperature. Since the temperature sensor is secured to a common base member on which the heating elements are mounted, it is impossible to measure the temperature variations of the individual heating elements. Additionally, there is a time lag between the output of the temperature sensor and the temperature value actually generated by the heating elements. Furthermore, the slow response characteristic of the temperature sensor is detrimental for precision control required in graphic applications.
In addition, recent needs for color printing have accelerated the demand for a printer capable of reproducing subtle gradations, and precision thermal energy control is thus required.